WHITBY, Ont. - A motorcycle passenger was killed Friday night in a disturbing accident on the busy 401 in which the body was hit by an unknown number of vehicles, none of which stopped to help.

The province's Special Investigations Unit is now probing the horrific death that shut down the highway for 12 hours.

Just after 11:30 p.m. a Durham police officer was following two people on a motorcycle in the westbound lanes of the highway, Ontario Provincial Police say.

The cop, who was driving a marked police cruiser, claims to have "signalled" the driver of the motorcycle to pull over, OPP Const. Graham Williamson said Saturday.

But instead of stopping, the motorcyclist took off.

As the bike accelerated, the passenger fell off the back, Williamson said. The passenger tumbled into the live lanes of traffic and is believed to have been hit by "one or more vehicles," he said. Remarkably, neither the motorcyclist nor any other driver remained at the scene to help.

The motorcycle passenger was pronounced dead on the highway.

The victim's name, age and gender were not immediately released.

The westbound 401 was shut down for more than 12 hours from Brock St. to Salem Rd. as the OPP and SIU conducted "parallel investigations." The SIU is called in any time there is a serious injury or death involving a police officer.

However, the spokesman for the provincial police watchdog could not be reached for comment.

The highway closure caused major traffic headaches for motorists throughout Durham Region.

The 401 finally reopened shortly after noon Saturday.

Investigators are trying to identify the motorcyclist and any other drivers who fled the scene, Williamson said.